Hydrogen powered polymer electrolyte membrane fuel cells (PEMFCs) are useful for power generation due to their high efficiency and power density, low emissions, low operating temperature, small size, and portability. Because of these characteristics, hydrogen PEMFCs are being developed for both mobile and stationary applications. Natural gas or methanol are more practical fuels than hydrogen for commercial devices because of storage and transportation issues. If natural gas or methanol is used as fuel sources for PEMFCs, a reforming step is first used to convert the fuel to hydrogen prior to entering the cell. Unfortunately, the carbon monoxide which is produced along with hydrogen in the reforming step degrades PEMFC performance.
The vast majority of published work addressing the catalytic oxidation of CO has not considered selectivity in the presence of hydrogen. In the early 1980s, Stark et al. reported that Pt supported on SnOx was an effective CO oxidation catalyst. Hoflund et al. also studied the Pt/SnOx system and compared it to Au/MnOx. They reported that Au/MnOx was more active and had a longer lifetime than Pt/SnOx. The most significant results reported were that the Au/MnOx catalyst produced nearly 60% conversion of CO to CO2 at only 35xc2x0 C. and increasing the temperature to 55xc2x0 C. produced over 80% conversion to CO2. Haruta et al. also have performed extensive testing on supported Au catalysts for oxidation of CO2. Using ultra-fine gold particles dispersed on the oxides of Fe, Co and Ni, they reported activity for the oxidation of CO at only xe2x88x9270xc2x0 C. Since Au alone is known to be inactive towards oxidation, and the metal oxides used in the above studies have very limited activity, the results obtained by Hoflund et al. and Haruta et al. underscore the synergistic effects between the metal and support material.
The catalytic properties of Ce-based catalysts also have been studied. CeO2 without any dopants or supported metals has been shown to be active for oxidation of CO with light-off temperatures above 300xc2x0 C. By adding La to promote oxygen vacancies, and a small quantity of Cu (1 at. %), either as bulk CuO or incorporated into the structure, the light-off temperatures for CO oxidation were reported to be reduced to less than 100xc2x0 C. Strong interactions between transition metal dopants and metal oxides are believed to be largely responsible for the enhancement in catalytic activity. Active sites formed by highly dispersed clusters of transition metal atoms and ions may promote oxidation at the phase boundary between the cluster and the metal oxide.
Selective CO oxidation in the presence of H2 has not been as thoroughly investigated as low-temperature oxidation activity. Oh and Sinkevitch studied performance for noble metals (Ru, Rh, Pt) supported on Al2O3. At high temperatures (xcx9c170xc2x0 C.) Ru/Al2O3 and Rh/Al2O3 were reported to demonstrate nearly 100% conversion of CO with approximately 5% conversion of H2 for optimum inlet O2 concentrations. These results were superior to those reported for Pt/Al2O3, which was reported to produce almost 20% conversion of H2 with 100% conversion of CO. At lower temperatures (xcx9c140xc2x0 C.) the better reported performance of Ru and Rh relative to Pt was even more pronounced, and Ru was significantly more selective than Rh. Kahlich et al. observed a similar trend between Ru/Al2O3 and Pt/Al2O3; however, the authors reported that Au/Fe2O3 generated comparable activities and selectivities at temperatures 70xc2x0 C. lower. Haruta et al. also noted high activity of Au/Fe2O3, as well as Au supported on Co3O4, NiO, Be(OH)2, and Mg(OH)2. Moreover, the authors reported that Au supported on selected metal oxides was more selective for CO oxidation relative to H2 than Pt and Pd catalysts. Furthermore, Bethke and Kung recently reported that selectivity for CO oxidation could be improved by decreasing Au particle size, and the authors suggest that the optimal particle size is between 5-10 nm. Finally, Sekizawa et al. reported that the water-gas shift catalyst Cu/Al2O3xe2x80x94ZnO has promise for selective removal of CO from methanol reformate under appropriate conditions. Despite these apparent successes, current catalysts for selective CO oxidation lack sufficient activity and selectivity at temperatures compatible with PEMFCs (i.e., around 90-110xc2x0 C.) and require careful control of O2 concentration in the feed for optimum performance.
Methods for removal of carbon monoxide from hydrogen fuel include adsorption, reduction and oxidation. Adsorption methods remove carbon monoxide by trapping it on a suitable substrate. Although this method is effective, a portion of the purified hydrogen stream must be used as a sweep gas to regenerate the adsorbent, which decreases the amount of fuel available for the cell. Furthermore, large quantities of adsorbent are needed and heat must be applied to the adsorbent to liberate carbon monoxide during regeneration. Alternatively, carbon monoxide can be removed by catalytic reduction to methane. Unfortunately, catalysts that promote this reaction also promote undesirable side reactions that result in generation of more carbon monoxide.
There is a need for catalysts which selectively remove CO from hydrogen-containing gases. One particular use for such catalysts is to remove CO from a reformate gas feedstream with minimal reduction in hydrogen content.
The catalysts in this invention are multi-component metal oxides with or without noble metals. These catalysts are useful to selectively remove carbon monoxide in the presence of hydrogen. The hydrogen may be present in a large excess. One specific application of the materials of the invention is improving fuel quality for hydrogen PEMFCs.
Catalysts described in this invention are believed to remove carbon monoxide by selectively oxidizing it to carbon dioxide. Carbon dioxide does not affect the performance of hydrogen PEMFCs. Furthermore, the catalysts described in this invention are distinct from other reported selective carbon monoxide oxidation catalysts since they do not rely on noble metals or metals supported on traditional carriers such as Al2O3. Some significant and distinct aspects of catalysts in this invention are that they are multi-component metal oxides tailored to give selective oxidation of carbon monoxide in the presence of hydrogen, including a large excess of hydrogen, and the compositions do not require expensive noble metals to be effective.
This invention relates to the composition, synthesis, and use of distinct metal oxide catalysts either with or without catalytic noble metals for selective oxidation of carbon monoxide in the presence of hydrogen.
The preferred catalyst compositions of the invention have the formula:
nN/Ce1xe2x88x92(x+y+z)AxAxe2x80x2yAxe2x80x3zO2xe2x88x92xcex4
where A, Axe2x80x2, Axe2x80x3 are independently selected from the group consisting of: Zr, Gd, La, Sc, Sr, Co, Cr, Fe, Mn, V, Ti, Cu and Ni; N is one or more members of the group consisting of Pt, Pd and Au;
n is a weight percent between 0 and 25;
x, y and z are independently 0 to 0.9;
x+y+z is 0.1 to 0.9; and
xcex4 is a number which renders the composition charge neutral. These catalyst compositions are also useful in the methods of the invention.
Other preferred catalyst compositions of the invention have the formula:
nN/(MOx)y(CeO2xe2x88x92xcex4)1xe2x88x92y, 
where
M is one or more members of the group selected from: Zr, Co, Cr, Fe, Mn, V, Ti, Ni and Cu; N is one or more members of the group consisting of Pt, Pd and Au;
n is a weight percent between 0 and 25;
y is 0.1 to 0.9;
and x and xcex4 make the compositions charge neutral. These catalyst compositions are also useful in the methods of the invention.
Catalyst compositions of the invention may comprise mixed oxides, single-phase materials, or multi-phase materials. Catalyst compositions further comprising a supporting material are also provided and are useful in the methods of the invention.
One class of compounds useful in the methods of the invention for selective removal of carbon monoxide in a hydrogen containing gas comprises Ce and at least one of ((a) or (b)) where (a) is one or more second metals selected from the group consisting of: Zr, Gd, La, Sc, and Sr; (b) is one or more third metals selected from the group consisting of: Co, Cr, Fe, Mn, V, Ti, Cu and Ni.
Another class of compositions useful in the methods of the invention are catalyst compositions with the formula:
nN/Ce1xe2x88x92xZrcAaAxe2x80x2axe2x80x2Axe2x80x3axe2x80x3BbBxe2x80x2bxe2x80x2Bxe2x80x3bxe2x80x3O2xe2x88x92xcex4
wherein n is a weight percentage from 0 to 25; N is one or more metals selected from the group consisting of Au, Pt, and Pd; x=a+axe2x80x2+axe2x80x3+b+bxe2x80x2+bxe2x80x3+c; a, axe2x80x2, axe2x80x3, b, bxe2x80x2, bxe2x80x3 and c are each, independently of one another, 0 to 0.9; xcex4 is a number which renders the composition charge neutral; A, Axe2x80x2 and Axe2x80x3 are independently selected from the group consisting of Gd, La, Sr and Sc; B, Bxe2x80x2 and Bxe2x80x3 are independently selected from the group consisting of Ti, V, Mn, Fe, Co, Cr, Ni, Au, Ag and Cu; provided that at least one of a, axe2x80x2, axe2x80x3, b, bxe2x80x2, bxe2x80x3 or c is nonzero.
In this class of compounds, preferably n is a weight percentage from 0 to 10, more preferably, n is a percentage from 0 to 5, and most preferably, n is a percentage from 0 to 3. In this class of compounds, preferably A, Axe2x80x2 and Axe2x80x3 are selected from the group consisting of Gd, La and Sr. In this class of compounds, preferably B, Bxe2x80x2 and Bxe2x80x3 are selected from the group consisting of Mn, Cu, Fe, Co and Cr. Preferably, c is between 0 to 0.2. Preferably, a+axe2x80x2+axe2x80x3 is between 0 and 0.1. Preferably b+bxe2x80x2+bxe2x80x3 is between 0.05 and 0.5.
Another class of compounds useful in the methods of the invention are those having formula:
nN/Ce1xe2x88x92xZrcAaAxe2x80x2axe2x80x2BbBxe2x80x2bxe2x80x2O2xe2x88x92xcex4
wherein n is a weight percentage from 0.01 to 15; N is one or more metals selected from the group consisting of Pt, Pd, and Au; x=a+axe2x80x2+b+bxe2x80x2+c; a, axe2x80x2, b, bxe2x80x2 and c are each, independently of one another, 0 to 0.5; xcex4 is a number which renders the composition charge neutral; A and Axe2x80x2 are independently selected from the group consisting of Gd, La, Sr and Sc; B and Bxe2x80x2 are independently selected from the group consisting of Ti, V, Mn, Fe, Co, Cr, Ni, Au, Ag and Cu; provided that at least one of a, axe2x80x2, b, bxe2x80x2 or c is nonzero.
In this class of compounds, preferably n is a percentage from 0.01 to 10, more preferably, n is a percentage from 0.01 to 5, and most preferably, n is a percentage from 0.01 to 3. In this class of compounds, preferably A and Axe2x80x2 are one or more of Gd, La and Sr, and preferably B and Bxe2x80x2 are one or more of Mn, Cu, Co, Cr and Fe.
Another class of compounds useful in the methods of this invention include those having formula:
nN/m Ce1xe2x88x92xAaAxe2x80x2axe2x80x2BbBxe2x80x2bxe2x80x2O2xe2x88x92xcex4/Zr1xe2x88x92zAxe2x80x3axe2x80x3Axe2x80x2xe2x80x3axe2x80x2xe2x80x3Bxe2x80x3bxe2x80x3Bxe2x80x2xe2x80x3bxe2x80x2xe2x80x3O2xe2x88x92xcex4
wherein n is a percentage from 0 to 15; m is a percentage greater than 0; N is one or more metals selected from the group consisting of Pt, Pd, and Au; x=a+axe2x80x2+b+bxe2x80x2; z=axe2x80x3+axe2x80x2xe2x80x3+bxe2x80x3+bxe2x80x2xe2x80x3; a, axe2x80x2, axe2x80x3, axe2x80x2xe2x80x3, b, bxe2x80x2, bxe2x80x3 and bxe2x80x2xe2x80x3 are each, independently of one another, 0 to 0.5; xcex4 is a number which renders the composition charge neutral; A, Axe2x80x2, Axe2x80x3 and Axe2x80x2xe2x80x3 are independently selected from the group consisting of Gd, La, Sr and Sc; B, Bxe2x80x2, Bxe2x80x3 and Bxe2x80x2xe2x80x3 are independently selected from the group consisting of Ti, V, Mn, Fe, Co, Cr, Ni, Au, Ag and Cu; provided that at least one of a, axe2x80x2, axe2x80x3, axe2x80x2xe2x80x3, b, bxe2x80x2, bxe2x80x3 or bxe2x80x2xe2x80x3 is nonzero.
In this class of compounds, preferably n is a percentage from 0 to 10, more preferably, n is a percentage from 0 to 5, and most preferably, n is a percentage from 0 to 3. Preferably m is a percentage from 0.5 to 25. In this class of compounds, preferably A, Axe2x80x2, Axe2x80x3 and Axe2x80x2xe2x80x3 are independently selected from the group consisting of Gd, La and Sr, and B, Bxe2x80x2, Bxe2x80x3 and Bxe2x80x2xe2x80x3 are independently selected from the group consisting of Mn, Cu, Co, Cr and Fe.
Another class of compounds useful in the methods of this invention include those having formula:
nN/m(CeO2)/p(AaAxe2x80x2axe2x80x2Axe2x80x3axe2x80x3BbBxe2x80x2bxe2x80x2Bxe2x80x3bxe2x80x3O2xe2x88x92xcex4)/q(Axe2x80x2xe2x80x3axe2x80x2xe2x80x3Axe2x80x3xe2x80x3axe2x80x3xe2x80x3Axe2x80x2xe2x80x3xe2x80x3axe2x80x2xe2x80x3xe2x80x3Bxe2x80x2xe2x80x3bxe2x80x2xe2x80x3Bxe2x80x3xe2x80x3bxe2x80x3xe2x80x3Bxe2x80x2xe2x80x3xe2x80x3bxe2x80x2xe2x80x3xe2x80x3O2xe2x88x92xcex4) 
wherein n, p and q are percentages from 0 to 50; m is a percentage greater than 0; N is one or more metals selected from the group consisting of Au, Pt and Pd; a, axe2x80x2, axe2x80x3, axe2x80x2xe2x80x3, axe2x80x3xe2x80x3, axe2x80x2xe2x80x3xe2x80x3, b, bxe2x80x2, bxe2x80x3, bxe2x80x2xe2x80x3, bxe2x80x3xe2x80x3 and bxe2x80x2xe2x80x3xe2x80x3 are each, independently of one another, 0 or 1; 67  is a number which renders the composition charge neutral; A, Axe2x80x2, Axe2x80x3, Axe2x80x2xe2x80x3, Axe2x80x3xe2x80x3 and Axe2x80x2xe2x80x3xe2x80x3 are independently selected from the group consisting of Gd, La, Sr and Sc; B, Bxe2x80x2, Bxe2x80x3, Bxe2x80x2xe2x80x3, Bxe2x80x3xe2x80x3 and Bxe2x80x2xe2x80x3xe2x80x3 are independently selected from the group consisting of Ti, V, Mn, Fe, Co, Cr, Ni, Au, Ag and Cu; provided that when n is zero, at least one of p and q is nonzero and at least one of a, axe2x80x2, axe2x80x3, axe2x80x2xe2x80x3,axe2x80x3xe2x80x3, axe2x80x2xe2x80x3xe2x80x3, b, bxe2x80x2, bxe2x80x3, bxe2x80x2xe2x80x3, bxe2x80x3xe2x80x3 and bxe2x80x2xe2x80x3xe2x80x3 is nonzero.
In this class of compounds, A, Axe2x80x2, Axe2x80x3, Axe2x80x2xe2x80x3, Axe2x80x3xe2x80x3 and Axe2x80x2xe2x80x3xe2x80x3 are preferably Gd, La and Sr, and B, Bxe2x80x2, Bxe2x80x3, Bxe2x80x2xe2x80x3, Bxe2x80x3xe2x80x3 and Bxe2x80x2xe2x80x3xe2x80x3 are preferably Mn, Cu, Co, Cr and Fe. Preferably, p and q are less than 50%. Preferably m is 10% to 50%. Preferably n is 5% or less.
More preferred catalysts of the invention include those having constituents with a preference for carbon monoxide. Such constituents include: copper, manganese and gold. Particularly useful materials include finely dispersed Au or Cu within composite metal oxides. Activity is promoted by high dispersion of these metals and synergy with the composite metal oxide support. Selectivity is achieved through preferential interaction of CO with polarizable Cu clusters and Au particles, as well as stabilized surface Cu ions. One class of compounds of the invention contains finely dispersed Au within composite metal oxides. Another class of compounds of the invention includes finely dispersed Cu within composite metal oxides. Useful catalysts of the invention include: Ce0.5Cu0.5Ow; Ce0.475Zr0.05Mn0.475Ow; Ce0.05Mn0.05Ow; Ce0.45Zr0.05Mn0.45Cu0.05Ow; and Ce0.5Fe0.1Cu0.4Ow. Other useful catalysts of the invention include: Mn0.5Fe0.5Ow; Ce0.1Mn0.45Cu0.45Ow; Ce0.1Mn0.45Fe0.45Ow; Ce0.3Mn0.7Ow; and Ce0.3 Mn0.65Zr0.05Ow. xcex4 and w are values which depend on the crystal structure of the composition, as known in the art and make the composition charge neutral. One class of compounds of the invention contains cerium at less than about 50 atomic percent of the metal components. Another class of compounds of the invention contains cerium at more than about 50 atomic percent of the metal components. Another class of compounds of the invention contains cerium at about 50 atomic percent of the metal components. Another class of compounds of the invention contains cerium at less than about 40 atomic percent of the metal components. Another class of compounds contains between 1-35 atomic percent CeO2. Another class of compounds of the invention contains cerium at less than about 30 atomic percent of the metal components. Another class of compounds of the invention contains cerium at less than about 25 atomic percent of the metal components. Another class of compounds of the invention contains cerium at less than about 65 atomic percent of the metal components. All intermediate ranges of cerium are included in the invention, as long as the compositions selectively remove carbon monoxide in the presence of hydrogen. Another class of compounds includes any composition of the invention with a low weight fraction of highly-dispersed gold, preferably less than 5 wt %, and more preferably less than about 1 wt %.
Also provided are methods for selectively removing carbon monoxide in a hydrogen-containing gas which comprise the step of contacting said gas with a catalyst composition of the invention. These methods may further comprise heating either said gas or said catalyst composition, or both, to a temperature sufficient remove the desired amount of carbon monoxide in said gas. The catalyst compositions may be held in a reactor at temperatures of from about ambient temperature to about 250xc2x0 C. Preferably, the temperatures are from about ambient temperature to about 150xc2x0 C. Preferably, temperatures from about 150xc2x0 C. to about 30xc2x0 C. are used. Most preferably, temperatures of about 120xc2x0 C. and below are used.
Catalyst compositions useful in the methods of the invention include those with a surface area ranging from about 20 to about 220 m2/g.
The catalyst compositions useful in the methods of the invention may be prepared by methods known in the art, or the methods described herein or modifications of methods known in the art or modifications of the methods described herein. The catalysts described in this invention can be prepared by coprecipitation, impregnation, precipitation deposition, ceramic processing, or hydrothermal processing techniques, or other methods known in the art.
Catalyst compositions of the invention include those with predominantly fluorite crystal structures. Other structures that may be present include defect fluorite, pyrochlore (A2B2O7) and perovskite-like phases, or phases resulting from other metal oxides in the catalyst such as oxides of Cu, Mn, Cr and Co. Cerium and zirconium oxide with some amount of dopants are generally present as fluorite structures. Dopants may also be present as oxides. A class of catalyst compositions of the invention are those that do not require an inert support, such as alumina or carbon. Another class of catalyst compositions of the invention include a support material such as a honeycomb matrix having inner and outer surfaces, wherein said catalyst material is present on the inner surfaces of said honeycomb matrix. Preferably, the support material is fabricated from ceramic materials, but may also be fabricated from metals or ceramic or metal fibers.
The catalyst compositions may be coated onto the support material by any method which produces a suitable coating of catalyst composition, including the method of: (a) treating a mixture of metal salt precursors with a precipitating reagent to form a precipitate; (b) preparing a slurry of said precipitate; (c) coating said slurry onto said support; and (d) calcining said slurry. The catalyst compositions may also be coated onto a support material by: (a) mixing a solution of metal salt precursors with the support; and (b) calcining said precursors. The catalyst compositions may also be coated onto said support material by: (a) mixing the support with one or more metal salt precursors to form a mixture; (b) treating said mixture with a precipitating reagent to form a precipitate; and (c) calcining said precipitate.
Also provided are methods of selectively removing carbon monoxide in a hydrogen-containing gas, comprising the steps of providing a reactor containing a catalyst composition of the invention; and passing the gas through the reactor to remove the carbon monoxide. Also provided is a catalytic reactor for selectively removing carbon monoxide from a hydrogen-containing gas which comprises: a casing having an entrance port, an exit port and a passage therebetween for the movement of said gases from said entrance port to said exit port with a catalyst composition of the invention in said passage. In the catalytic reactor, the gases preferably contact said catalyst before exiting said casing.
As used herein, xe2x80x9ccatalyst compositionxe2x80x9d includes those compositions useful for selective removal of carbon monoxide in a hydrogen-containing gas. As used herein, xe2x80x9cmixed metal oxidesxe2x80x9d include one or more metal oxides. As used herein, xe2x80x9csingle-phase materialxe2x80x9d is a material that comprises a single crystallographic phase. As used herein, xe2x80x9cmulti-phase materialxe2x80x9d refers to a material wherein some components are single-phase and other components are mixed metal oxides. As used herein, a xe2x80x9cprecipitating reagentxe2x80x9d is a substance or mixture of substances that causes precipitation of a desired substance. Preferred precipitating reagents include NH4OH, (NH4)2CO3, Na2CO3, NaOH, urea and K2CO3. As used herein, xe2x80x9ccontactingxe2x80x9d substances is meant to indicate that substances are physically near each other, but is not intended to mean a homogeneous solution is formed.
The catalysts of this invention are suitable for use in any reactor system and particularly with either fixed and fluid bed reactors and can be prepared as powders or pressed into plugs, pellets and other shapes suitable for use in a given reactor configuration.
The catalyst compositions of the invention may be used to reduce the concentration of carbon monoxide in a gas mixture. The components of the gas mixture may include methane, carbon dioxide, carbon monoxide, oxygen, water, nitrogen, argon, native components of air, hydrogen and other hydrocarbons, or any mixture of the foregoing. The gas may also include other substances, as known in the art, for example other components of a fuel cell feed. The gas mixture must contain some oxygen, preferably not a large excess of oxygen. The gas mixture preferably contains a stoichiometric amount of oxygen. The catalysts of the invention function in the presence of potentially interfering substances, such as water, sulfur-containing gases and halogens.
The catalysts are preferably preconditioned prior to said gases contacting said catalyst. The preconditioning treatment is useful to desorb moisture and change the oxidation state of some species. More preferably, the catalysts are preconditioned at a temperature of between about 150xc2x0 C. to 400xc2x0 C. Catalyst compositions of the invention are preferably preconditioned under a flow of air for a time sufficient to maximize activity, preferably for one hour or more. The preferred preconditioning time is longer at lower temperatures and can be as long as 24 hours at temperatures of 100xc2x0 C. or less.
Catalysts of the invention have long lifetimes and can be regenerated by heating for a sufficient time to drive off adsorbed organics and moisture. For example, catalysts of the invention may be regenerated by heating at a temperature of about 150xc2x0 C.
The catalysts can contain between 0 and 25 weight percent of Pt, Pd and Au, or combinations of these metals and any intermediate value therein. Preferably, the amount of Pt, Pd and Au is as small as possible, because the metals are expensive. Preferably, less than about 10 weight percent of these metals are present. If present, Pt and Au are in the metallic state and Pd can be in the metallic state or as an oxide.
As used herein, xe2x80x9cselective removal of carbon monoxide in the presence of hydrogenxe2x80x9d means that more carbon monoxide is removed from a gas than hydrogen is removed. It is preferred that as much carbon monoxide is removed as possible, while the level of hydrogen is maintained to as great an extent as possible. The carbon monoxide does not need to be all removed for the removal to be encompassed by the term, as long as the carbon monoxide is reduced more than the hydrogen is reduced. The desired amount of carbon monoxide which is removed is dependent on the particular application of the catalyst or method. The amount of carbon monoxide which is removed is dependent on the temperature of the reaction, the catalyst composition, the flow rates, the composition of the inlet stream, and other parameters as known in the art. One particular example of selective removal is shown in FIG. 2. In some applications, it is preferred that the concentration of carbon monoxide be removed to less than 10 ppm in the gas mixture. All catalyst compositions of this invention are useful to selectively remove carbon monoxide from a hydrogen-containing gas.